This invention relates to an apparatus for producing a failure signal in the event of failure of a steering handle position sensor which may be used in controlling vehicle dynamic characteristics including yaw velocity, side velocity, roll velocity, etc. Although the invention will be described in connection with a rear wheel steering control system, it should be understood that the invention is equally applicable to other vehicle dynamic characteristic control systems including active suspension control systems and the like utilizing information on steering handle position to control the dynamic characteristics of motor vehicles.
For example, U.S. patent application Ser. No. 07/284,414, filed Dec. 14, 1988, now abandoned, discloses a rear wheel steering control system for controlling the angle of steering of the rear wheels based on a rear wheel steering angle calculated as a function of vehicle speed and front wheel steering angle. For the purpose of calculating the front wheel steering angle, which corresponds to a deviation of the existing steering handle position from a steering handle neutral position indicating a driver's demand for straight ahead driving, the rear wheel steering control system includes a steering handle position sensor for sensing the existing steering handle position and a steering handle neutral position sensor for producing a steering wheel neutral position signal having a first level when the steering handle position is within a predetermined range and a second level when the steering handle position is out of the predetermined range. The rear wheel steering control system estimates the steering handle neutral position by calculating an average value of the steering wheel position sensed when the steering wheel neutral position signal changes from the second level to the first level and the steering handle position sensed when the steering wheel neutral position signal changes from the first level to the second level. Japanese Patent Kokai No. 59-26341 also discloses an apparatus which employs a steering handle position sensor and a steering handle neutral position sensor to estimate a steering handle neutral position. The estimated steering handle neutral position is used to provide information used in controlling a four wheel steering system.
With such conventional control systems, however, the estimated steering handle neutral position will be diviated to a great extent from a correct neutral position, causing the rear wheels to be turned in response to a driver's demand for straight ahead driving when a failure occurs in the steering wheel position sensor or the circuit associated therewith to maintain the steering handle neutral position signal at one of the first and second levels.
An indication of failure in the steering handle position sensor or the circuit associated therewith has been provided when the two conditions are fulfilled, that is, when the vehicle speed exceeds a predetermined value, for example, 60 Km/h, and the steering handle position signal produced from the steering handle position does not indicate a steering angle equal to or greater than a predetermined value, for example, 1 degree, for a predetermined interval of time, for example, 180 sec. However, controls will be made based upon the steering handle position sensed when the steering handle position sensor or the circuit associated therewith is subject to failure until the predetermined period of time has been elapsed. That is, the conventional failure indication has a slow response to an event of failure of the steering handle position sensor. Furthermore, the failure indication cannot be produced when the vehicle is running at a speed, about 50 Km/h, which is normal for vehicle running in an urban area.